Method, device and computer product for making an individual model of a jaw-bone

ABSTRACT

The invention relates to a method, a device and a computer software for making an individual model of a jawbone, which are particularly useful in the development of dental implants or pre-operative planning of implants. The method comprises the following steps: making a tomogram of a jawbone perpendicular to the longitudinal axis of the jawbone; determining the boundary of the jawbone and the surrounding tissue and the boundary between the compact and spongiosal portion of the jawbone; transforming said boundaries into a two-dimensional profile; and extruding the two-dimensional profile into a three-dimensional volume body model of the jawbone. The volume body model of the jawbone may be combined with the volume body model of an implant and then transformed into a finite element model.

The present invention relates to a method, a device and a computer software for making an individual model of a jawbone, which is particularly useful in the development of dental implants or pre-operative planning of implants.

The success of an implant insertion depends decisively on the distribution of chewing forces and stresses that are introduced via the implant into the jawbone. Besides shape and size of the implant the individual shape and composition of the jawbone is an important factor.

From the prior art, various finite element models (FE models) of jawbones are known, but these models do not stand the test in practice unconditionally.

For example, Farah J, Craig R and Meroueh K: “Finite element analysis of a mandibular model”, Journal of Oral Rehabilitation 16, 603-11 (1989) suggests directly constructing a two-dimensional model of a lower jaw in an FE program (see attached FIG. 14). This method makes the attempt to copy the complex configuration of the jawbone as good as possible in a two-dimensional way, but it does not result in a morphological exact transfer of the configuration into the three-dimensional space. Due to the simplification of the complex geometry, this method is at best useful for solving basic problems. Individual morphological features, however, are left unconsidered. What is more, the manual construction of the model is time-consuming, tiresome and error-prone.

On the other hand, van Zyl P, Grundling N, Jooste C and Terblanche E: “Three-dimensional finite element model of a human mandible incorporating six osseointegrated implants for stress analysis of mandibular cantilever prostheses”, International Journal of Oral Maxillofac Implants 10, 51-7 (1995) describes a three-dimensional FE model of a lower jawbone which is not constructed but which is generated from tomograms of a real lower jaw (see attached FIG. 15). In this method, a point cloud obtained from the tomograms is directly input into a FE program where the points of the point cloud are used as nodes for the FE model. The current FE programs, however, have difficulties with making a three-dimensional network from dense point clouds, so that this approach works only in certain cases while mostly losses in the representation of the complex morphological configuration have to be accepted. As a rule, the FE program must therefore be adapted to the specific application under a high expenditure of time, a measure which can only be carried out by experts and results in isolated applications.

The object of the invention is to provide a method, a device and a computer software capable of generating an individual model of the jawbone, with relatively low expenditure of time and good accuracy.

The above object is solved, for example, by a method in which a tomogram of the jawbone is made in a direction perpendicular to the longitudinal axis of the jawbone and which comprises the following image processing steps: i) determining the boundary of the jawbone and the surrounding tissue based on the tomogram; ii) transforming the boundary between the jawbone and the surrounding tissue into a two-dimensional profile; and iii) extruding the two-dimensional profile to obtain a three-dimensional volume body model of the jawbone.

The technical term “extruding” means that the two-dimensional profile is extended in a certain direction (i.e. in the direction of the longitudinal axis of the jawbone) to give volume to the model. In other words, the profile is drawn along a path until a predetermined end position is reached. The extrusion path may run along the surface normal given by the two-dimensional profile, but it may also deviate from the normal and be adjusted closer to the exact run of the longitudinal axis of the jawbone.

The method according to the invention takes advantage of the fact that the jawbone does hardly change its inherently complex configuration in the longitudinal direction over a short distance such as across a gash.

For developing dental implants, usually a model of the jawbone having a length corresponding to a gash (having a width of about 5-10 mm) is sufficient. Therefore, it is fully adequate if only a single tomogram of the jawbone is made to obtain the two-dimensional profile, which is then extruded in the direction of the longitudinal axis of the jawbone across the width of the gash to obtain the three-dimensional volume body model. Thus, the proposed method gets by with relatively few image data. That is why the image processing step i) to iii) can be executed fast and time-saving. Nevertheless, the method is sufficiently exact for the individual features of the jawbone be taken into account.

If the accuracy of the volume body model is to be increased or if the configuration of the jawbone is to be reproduced over a longer distance, it is also possible to make a plurality of tomograms in the direction of the longitudinal axis of the jawbone and to generate the three-dimensional volume body model by extruding across the respective profiles obtained thereby. In this case, the volume body model spans the respective profiles and has a cross section that changes in accordance with the profile outline.

If the composition of the jawbone is of particular importance for respective fields of application of the model, the boundary between the spongiosal and compact portion of the jawbone may be determined in addition to the boundary of the jawbone and the surrounding tissue to generate a two-part volume body model of the spongiosal and compact jawbone.

The volume body model of the jawbone may be transformed into the three-dimensional finite element model (FE model) either directly or after combining it with the volume body model of a dental implant and/or tooth. Combining the volume body model with the dental implant and/or tooth is a further image processing step iv). The geometry of a volume body model can be made into a high-resolution FE network with relative ease, so that not much time is required for generating the FE model.

If different material properties are assigned to the respective parts of the FE model (i.e. the jawbone and the dental implant or tooth), complex calculations can be made by means of the FE model, for example, relating to the distribution of stress in the jawbone when a force acts on the dental implant and/or the tooth.

The above method may be carried out, for example, by using a device which comprises an equipment for making a tomogram of a jawbone in a direction perpendicular to the longitudinal axis of the jawbone, and a data processor for executing the following data processing steps: reading and, if need be, digitizing one or a plurality of tomograms of the jawbone made by the equipment; and executing the above-mentioned image processing steps i) to iii) and optionally iv) by means of CAD techniques (CAD: computer aided design) to generate a three-dimensional volume body model of a jawbone or a jawbone combined with a dental implant and/or tooth. Concerning the image processing step iii) in which the two-dimensional profile is extruded to obtain the three-dimensional volume body model, in particular the CAD techniques “extruding”, “sweeping” and “lofting” come into question.

The volume body models generated by means of the CAD techniques may be transformed by CAD-FEM-coupling into individual FE models.

It goes without saying that the invention may be realized, instead of using the above device, by a computer software which implements the above-mentioned data processing steps based on software routines when running on a computer. The computer software may be stored on a data carrier or may be directly loaded into the working memory of the computer.

The data processing steps may be automated to the greatest possible extent, so that the invention is made available even to a non-professional who has no experience in the field of CAD or FE programs. Moreover, automation avoids inaccuracies and errors inevitable in case a model is directly constructed. Owing to the fact that the invention may use software routines known from current CAD and FE programs, it is possible to avoid isolated applications.

The above and further solutions to the object of the invention as well as the features and advantages thereof are now discussed in more detail by referring to specific embodiments of the invention. The description of the embodiments is to be read with the accompanying drawings which show the following:

FIG. 1 an X-ray tomogram in a direction perpendicular to the longitudinal axis of an upper and lower jaw;

FIG. 2 a magnified section of the X-ray tomogram of FIG. 1 in which the boundary of the spongiosal and compact lower jawbone is marked;

FIG. 3 a two-part profile of the spongiosal and compact bone, which has been generated from FIG. 2;

FIG. 4 a wire frame image of a three-dimensional volume body obtained by extruding the profile of FIG. 3;

FIG. 5 a surface image of the volume body model of FIG. 4;

FIG. 6 a photography of a cylinder-type implant;

FIG. 7 a half profile of the implant shown in FIG. 6;

FIG. 8 a wire frame image of a three-dimensional volume body model obtained by rotating the half profile shown in FIG. 7 around the center line by 360°;

FIG. 9 a surface image of the volume body model of FIG. 8;

FIG. 10 a surface image of a three-part volume body model obtained by combining the volume body models shown in FIGS. 5 and 9;

FIG. 11 a three-dimensional FE model of a spongiosal and compact lower jawbone with an implant;

FIG. 12 a surface image of a three-dimensional volume body model of a disc-type implant;

FIG. 13 a surface image of a three-dimensional volume body model of a sheet-type implant;

FIG. 14 a two-dimensional FE model of a lower jaw half according to the state of the art; and

FIG. 15 a three-dimensional FE model of a lower jaw according to the state of the art.

In the following, an application is described in which an individual model of a patient's lower jaw is generated and the lower jaw model thus generated is combined with a model of a cylinder-type implant. The steps for generating the lower jaw model and the implant model and the way of combining both models are discussed separately.

Generating the Lower Jaw Model

Using an equipment for making a tomogram in the vicinity of a gash where insertion of an implant is planned, a tomogram is made in a direction perpendicular to the longitudinal axis of the lower jawbone. FIG. 1 shows such a tomogram transversal to the longitudinal axis of the lower jaw, which has been made by using a digital volume tomography apparatus (DVT).

Since the obtained tomogram is already digital, it can be imported without previous scanning into a CAD program (e.g. Mechanical Desktop™ of Autodesk Inc.), which runs on a data processor such as a work station. The tomogram is scaled such that the dimensions in the CAD program correspond to the dimensions of the tomogram.

In the CAD program a two-dimensional sketch of the spongiosal and compact lower jawbone is prepared, with the tomogram serving as a drawing pattern in the background. As shown in FIG. 2, at that time the boundaries between the compact portion (light gray) of the bone and the surrounding tissue (dark gray) and between the spongiosal portion (dark gray) and the compact portion (light gray) of the bone are marked with lines, polylines, or splines.

After having served as a drawing pattern, the tomogram may be cleared. The remaining two-dimensional sketch is further processed and converted into a two-dimensional profile as shown in FIG. 3. The tolerance values of the CAD program (for the angle etc.) are preferably set such that a one-to-one conversion is obtained which is geometrically as exact as possible.

Afterwards, the two-dimensional profile is extruded, as shown in FIGS. 4 and 5, along the surface normal given by the profile to be transformed into a three-dimensional volume body model. The extrusion length depends on the patient's gash and is in the present case 10 mm.

The volume body model of the lower jawbone thus obtained consists of two parts, namely an inner part defining the spongiosal portion of the bone and an outer part defining the compact portion of the bone. The volume body model is a good approximation of the individual shape and composition of the jawbone in the vicinity of the gash.

Generating the Implant Model

At first, a photography of the cylinder-type implant to be used for implant insertion is taken. FIG. 6 shows a longitudinal view of the implant.

The photography is scanned into the above-mentioned CAD program and is scaled to the size of the lower jaw image data. Then, the outline of the implant is marked in the CAD program by lines, polylines or splines, with the photography being in the background. For reasons of simplicity, only half of the outline is taken, resulting in the half profile sketch shown in FIG. 7.

After the photography has been cleared, the half profile sketch of the implant is converted into a profile which is transformed into a three-dimensional volume body model by rotating it around the center line by 360°. Contrary to the real implant, this volume body model, which is shown in FIGS. 8 and 9, is axially symmetric. Neglecting the lead, however, is irrelevant for the calculations described below.

Combining the Lower Jaw Model with the Implant Model

The volume body model of the cylinder-implant shown in FIG. 9 is subtracted from the volume body model of the lower jawbone shown in FIG. 5 by applying Boolean-difference operation. Then, the volume body model of the cylinder-type implant is inserted into the resulting recess. By doing so, the three-dimensional volume body model shown in FIG. 10 is obtained, in which the spongiosal and compact bone is combined with the implant.

Then, the three-part volume body model thus obtained is read via a CAD-FEM interface (e.g. by using the instruction AMACISOUT in the program Mechanical Desktop™ of Autodesk Inc.) into an FE program (e.g. Design Space™ of Ansys Inc.). As shown in FIG. 10, the complex geometry of the three-part volume body model is transformed by the FE program into a high-resolution three-dimensional FE network.

At the time of generating the FE model, different material properties (e.g. different coefficients of elasticity) are assigned to the spongiosal and compact portion of the lower jawbone and the dental implant. After having determined the bearings and loads, the distribution of stress as well as deformation and strain in the jawbone can be simulated for the case that a force (e.g. a chewing force) is introduced via the implant into the jawbone.

The results obtained from the above FE model can be used for checking the compatibility of the implant with the individual shape and composition of the patient's jawbone. If, for example, a particular large compressive stress develops in certain areas of the jawbone, there is the risk that the bone will be decomposed in these areas after implant insertion, thus impairing the long-term prognosis for the implant. In this case it should be checked whether an implant having a different size or shape offers a better prognosis. FIGS. 11 and 12 exemplify further volume body models of a disc-type implant and a sheet-type implant, which have been reproduced from a photography in a similar way as the volume body model of the cylinder-type implant shown in FIG. 9 and which may be combined with the volume body model of the lower jawbone shown in FIG. 5 to obtain comparative results.

The results obtained from the above FE models may also be used for manufacturing implants that are specifically adapted to the individual jaw anatomy so as to transmit the chewing force to the jaw in an optimal way. The results may also be used for optimizing ready-made implants as well as for manufacturing individual, non-ready-made implants.

Modifications

Since the upper jaw does hardly change its complex configuration in the direction of its longitudinal axis across a short distance, the above teaching may also be applied to the upper jaw.

Further, the tomogram of the jawbone may be taken by an equipment other than a digital volume tomography (DVT) apparatus. Examples for such an equipment are, for example, conventional or spiral-type computer tomography (CT) apparatuses, nuclear magnetic resonance (NMR) apparatuses, or sonographic apparatuses. In principle, images of histologic sections can be used as well.

When the tomogram of the jawbone is not digital, the tomogram may be digitized by scanning or the like and be read into the data processor for further processing. However, the bone boundaries may also be determined graphically by counterdrawing or the like, and then digitizing the sketch thus obtained and reading it into the data processor.

Instead of tracing or counterdrawing the bone boundaries manually, the bone boundaries may automatically be detected by the data processor based on the existing difference in brightness value. By doing so, it is possible to improve the reproducibility of the results.

In the above-mentioned application, only a single tomogram was used for generating the jawbone model to minimize the work load and the computational effort. However, it is also possible to take several tomograms with a known spacing therebetween, and generating several profiles therefrom. Then, the respective profiles are aligned in a CAD program with the known spacing in an anatomically correct way. Thereafter, the three-dimensional volume body model of the jawbone is generated by expanding or lofting the respective profiles, with the resulting volume body model being spanned over each profile.

Apart from that, it is possible, although more cumbersome, to generate two separate FE models of the jawbone and the implant at first and then to combine them in the FE program.

The model of the jawbone may also be combined with models other than the implant model such as with a model of a tooth. Furthermore, it is possible to extend the model of the jawbone combined with the implant beyond the area of the gash, so that the teeth adjacent to the gash are included.

The overall process can be simplified by using a device which is specifically adapted to the application, in which the image processing steps of the data processor are adjusted to the tomograms taken by the equipment and in which the data processing steps of the CAD program are linked and automated to the greatest possible extent. In such a device it is not necessary that the equipment and the data processor are directly coupled with each other, nor it is necessary that the data processing steps are executed in one and the same data processor. Instead of a closed system, it is also possible that the treating dentist sends the tomogram(s) of the patient or the finished jaw model to the manufacturer of the implants who selects or produces a compatible implant on this basis accurate to size. If, on the other hand, the implant selection is on the side of the treating dentist, the dentist could be supplied by manufacturers with design data or models of the available implants.

The above-mentioned FE models of the jawbone combined with the implant and/or tooth may not only be used for calculating the distribution of stress or strain and deformation, but also for calculating distributions of temperature in case there is a temperature difference across the implant, or for a frequency analysis in case of an ultrasonic calculus treatment on the implant or tooth.

The method, the device and the computer software for generating the jaw model are useful not only in the development of dental implants or pre-operative planning of implants, but the volume body models and FE models of the jawbone generated as discussed above may also be used in principle for illustrating and simulating oral surgery, for adjusting dentures seated on the jawbone, or for planning corrections of defective dental positions. A further application is generating models of a fractured jawbone by combining the three-dimensional model of the jawbone with three-dimensional models of metal plates and screws used for stabilizing the fracture. 

1. A method for generating an individual model of a jawbone segment, wherein only one tomogram of the jawbone segment is made in a direction perpendicular to the longitudinal axis of the jawbone segment, said method comprising the following image processing steps: i) determining the boundary of the jawbone and the surrounding tissue based on the tomogram; ii) transforming the boundary between the jawbone and the surrounding tissue into a two-dimensional profile; and iii) extruding the two-dimensional profile in the direction of the longitudinal axis of the jawbone segment to obtain a three-dimensional volume body model of the jawbone segment.
 2. A method according to claim 1, wherein the boundary between the spongiosal and compact portion of the jawbone is determined in addition to the boundary of the jawbone and the surrounding tissue to generate a two-part volume body model of a spongiosal and compact jawbone segment.
 3. A method according to claim 2, further comprising generating a three-dimensional finite element model from the two-part volume body model of the spongiosal and compact jawbone segment.
 4. A method according to claim 2, further comprising the following image processing step: iv) combining the volume body model of the jawbone segment with a volume body model of a dental implant and/or tooth to obtain a volume body model of the jawbone segment combined with the dental implant and/or tooth.
 5. A method according to claim 4, further comprising generating a three-dimensional finite element model from the volume body model of the jawbone segment combined with the dental implant and/or tooth.
 6. A method according to claim 5, wherein different material properties are assigned to the jawbone segment and the dental implant or tooth when generating the three-dimensional finite element model.
 7. A method according to claim 6, further comprising calculating, by means of the finite element model, the distribution of stress in the jawbone when a force acts on the dental implant and/or the tooth.
 8. A device for carrying out the method according to any one of claims 1 to 7, comprising: an equipment for making a tomogram of a jawbone segment in a direction perpendicular to the longitudinal axis of the jawbone segment; and a data processor for executing the following data processing steps: reading and, if need be, digitizing a tomogram of the jawbone segment made by the equipment; and executing the image processing steps of the method by means of CAD techniques to generate a three-dimensional volume body model of the jawbone segment or the jawbone segment combined with the dental implant and/or tooth.
 9. A device according to claim 8, wherein the data processor generates a three-dimensional finite element model from the volume body model of the jawbone segment or the jawbone segment combined with the dental implant or tooth by CAD-FEM-coupling.
 10. A device according to any one of claims 8 or 9, wherein the data processor implements the data processing steps based on software routines.
 11. A computer software which carries out the method according to any one of claims 1 to 7 by implementing the following data processing steps based on software routines when running on a computer: reading and, if need be, digitizing a tomogram of a jawbone segment made in a direction perpendicular to the longitudinal axis of the jawbone segment; and executing the image processing steps of the method by means of CAD techniques to generate a three-dimensional volume body model of the jawbone segment or the jawbone segment combined with the dental implant and/or tooth.
 12. A computer program according to claim 11, which generates a three-dimensional finite element model from the volume body model of the jawbone segment or the jawbone segment combined with the dental implant or tooth by CAD-FEM-coupling. 